This invention relates to impressioning keys, that is to say key blanks which are used to make an impression of the tumblers in a lock so that the lock can be opened and a permanent key made from an impressioned blank.
In theory, a conventional metal key blank can be used as an impressioning key by inserting the blank into a lock, turning the blank until the tumblers are engaged, and then moving the key up and down transversely to its length so that the tumblers produce an impression on the blank. However, the impressions obtained on a conventional key blank are usually barely perceptible and could more properly be described as shine marks. It is thus almost impossible even for a skilled locksmith to "read" the marks and appropriately file away the blank in the region of the marks to produce a first stage impression key, and of course the process has to be repeated until the impressioned key opens the lock. Additionally, a user must develop the skill to file away only one depth increment at a time. Too small a depth means that more filing must be carried out, and too great a depth ruins that particular blank for its intended purpose.
In practice, it has been known for many years to remove the tumbler engaging portion of a key blank and replace the removed portion with solder, usually soft solder with a composition of about 50% tin, 50% lead. Because of the nature of solder, especially its low melting point of about 230.degree. F., a large lump of solder becomes applied to the cut-away blank, and thus large amounts have to be filed away to the shape of the removed portion. Not only is this a tedious procedure, but filing solder quickly clogs up and thus destroys a file. The solder cannot be pre-shaped since it must be melted to weld it to the blank. Also, solder is relatively expensive, and this known procedure is very wasteful of the material.